1. Field
This disclosure relates to non-volatile storage.
2. Description of the Related Art
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to the traditional, full-featured EEPROM.
Both traditional EEPROM and flash memory may utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage (VTH) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
One possibility is to store a single bit in a each memory cell. All memory cells in a group may be erased, and then those to receive a “1” may be programmed such that the threshold voltage is above a program verify threshold voltage. The state of a memory cell can later be determined by reading the memory cell to determine whether its threshold voltage is above or below a read reference threshold voltage. The read reference threshold voltage may be somewhat less than the program verify threshold voltage to provide a margin of error. It is also possible to store multiple bits per memory cell by programming a given memory cell to one of a number of possible threshold voltage ranges. For example, if memory cells are programmed to one of four threshold voltage ranges, then each memory cell could store two bits. Eight threshold voltage ranges might be used to store three bits per memory cell, and so on.
In some architectures there are word lines and bit lines. The word lines may be used to apply a voltage to the control gates when programming and reading memory cells. For example, during a read operation, a read reference voltage may be applied to the control gates of selected memory cells, and a read pass voltage may be applied to control gates of unselected memory cells. A bit line that is associated with a given selected memory cell may be sensed to determine how the selected memory cell responds to application of the read reference voltage. In one technique, the selected memory cell will conduct a significant current if its threshold voltage is less than the read reference voltage.
In one possible approach for reading memory cells that store multiple bits, several read reference voltages of different magnitudes are applied to the selected word line. Note that there may be a delay between the time that a read reference voltage is first applied to the selected word line and the time that the bit line may be sensed. One reason for this delay is that, for an accurate reading, the voltage on the selected word line may need to stabilize before the bit lines can be sensed. That is, all portions of the selected word line should be at the read reference voltage prior to sensing the bit lines. Because there may need to be several read reference voltages of different magnitudes to complete the read operation, such delays may impact overall sensing time. For example, in one approach seven different read reference voltages are applied to determine which of eight states a memory cell is in. In one approach, 15 different read reference voltages are applied to determine which of 16 states a memory cell is in.